Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles:  A Comparison between Sodium and Potassium Counterions

Abstract

We present the results of a molecular dynamics simulation study of the effects of counterion type on the properties of the interior region of aqueous reverse micelles. The model systems, which treat only the interior region at the atomistic level, are designed to represent water-in-oil microemulsions formed by the aerosol-OT surfactant, either with its usual counterion, Na + , or with the K + counterion. Our study covers the water content, w0 ) [H2O]/[surfactant], range of 1-7.5, where the reverse micelles are approximately spherical and contain tens to hundreds of water molecules in their interior pool. We find that several key structural and dynamical features of the reverse micelle water pool are strongly affected by counterion type. These effects can be ascribed to the differences in headgroup-counterion coordination and to the stronger affinity for water that the smaller Na + ion exhibits. At low water content, K + ions are able to coordinate four headgroups, while Na + coordinates a maximum of three. As w0 increases, this coordination number decreases for both ions but always remains higher for K + , which also has a stronger tendency to remain in the interfacial region than does Na + . K + displaces water from the interface to a larger extent than does Na + , with the result that fewer water molecules are trapped between the headgroups. That and the fact that K + has a weaker attraction for water than does Na + lead to higher mobility of water throughout the reverse micelle interior and more bulklike structural features, such as the number of water-water hydrogen bonds, in the interfacial region.

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

“…[13-16, 31, 37-40, 58] In the present RMs, the Na + counterion can accommodate six coordinating water molecules in its solvation shell. [15,38,39,59] Therefore, the number of water molecules required to fully solvate an AOT head group and its counterion in RMs is certainly much more than three. [26,28,40,42,60] FTIR studies by Jain et al [42] indicated that the AOT molecule can be hydrated by a maximum of 12 water molecules.…”

Impact of Surface Active Compounds on Iron Catalyzed Oxidation of Methyl Linolenate in AOT–Water–Hexadecane Systems

“…Experimental and modeling studies 22,34,[36][37][38][39] suggest that there is a layered water pool structure where some water molecules are associated with surfactant head groups, some form an intermediate layer consisting of the next few nearest neighbors and, in the case of large reverse micelles, an interior region containing bulk water. From this standpoint, it is possible that, in the reverse micelles with low amounts of added water, most of the water molecules strongly associated with the AOT head groups.…”

Molecular dynamics simulations of cytochrome c unfolding in AOT reverse micelles: The first steps

“…Recently, molecular dynamic simulations (MD) have been successfully used to gather additional information about AOT RM structure and dynamics at the atomic level [9][10][11][12][13][14][15][16] . These studies have underlined the effect of the charged, restricted microenvironment on the confined water structural 13,14,16 and dynamical properties 13,14,17 existing in RM.…”